Controller of an internal combustion engine

ABSTRACT

When the first fuel injection is performed after reversion from the fuel cut, the fuel pulse width Ka·INJ.PW is set so that a fuel supply amount is greatly increased in relation to an intake air amount, and the ignition timing is set to the first retarded ignition timing θa. When the second and subsequent fuel injections are performed, the fuel pulse width Kb·INJ.PW that is smaller in increase width of fuel is set, and the ignition timing is set to the second retarded ignition timing θb that has a retardation amount smaller than that of the first retarded ignition timing θa.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a controller of an internal combustionengine, and more specifically to the control that is implemented whenfuel cut is finished to restart fuel supply in an internal combustionengine.

2. Description of the Related Art

In recent years, it is widely known to stop fuel supply to each cylinderof an internal combustion engine, that is, to perform so-called fuelcut, at the time of deceleration or the like of a vehicle. During fuelcut, combustion is not produced in engine cylinders, and the fresh airsucked in by the engine flows directly through an exhaust passage intoan exhaust gas purification catalyst. Therefore, a large quantity ofoxygen is adsorbed on the exhaust gas purification catalyst.

By the time the engine returns from a fuel cut condition to a fuelsupply condition, the exhaust gas purification performance is degradeddue to the adsorption of a large quantity of oxygen on the exhaust gaspurification catalyst. This causes the problem that the discharge amountof NOx (nitrogen oxides) into the atmosphere is increased. Theregenerates another problem that, shortly after reversion from fuel cut,the brake horsepower of the engine is suddenly changed to cause shock.

In respect of the former problem, there has been developed thetechnology of early consuming the oxygen adsorbed on the exhaust gaspurification catalyst by increasing compensation amount of a fuelinjection amount to set an exhaust gas air-fuel ratio to the vicinity ofa stoichiometric air-fuel ratio in the time period from the reversionfrom fuel cut to the implementation of air-fuel ratio feedback control.This technology is disclosed, for example, in Unexamined Japanese PatentApplication Publication No. 2003-254126 (hereinafter, referred to asPatent Document 1).

As to the latter problem, there has been developed the technology ofpreventing shock by reducing an intake air amount and retarding theignition timing at the time of reversion from fuel cut. This technologyis disclosed, for example, in Unexamined Japanese Patent ApplicationPublication No. 6-288327 (hereinafter, referred to as Patent Document2).

The technology disclosed in Patent Document 1, however, has not beendeveloped in consideration of the shock that is caused along with thereversion from fuel cut.

According to the technology disclosed in Patent Document 2, the misfirelimit of ignition timing retardation in normal control of the internalcombustion engine is at around zero degree BTDC. For this reason, evenif the ignition timing is retarded to the misfire limit, it is hard toreduce the shock sufficiently. In addition, there are problems includingone that, since the oxygen adsorption on the exhaust gas purificationcatalyst during fuel cut is not taken into account, the discharge amountof NOx into the atmosphere is increased, which is an undesirablecondition.

SUMMARY OF THE INVENTION

An aspect of the present invention is a controller of an internalcombustion engine capable of carrying out fuel cut to stop fuel supplyinto each cylinder, the controller comprising: an exhaust gaspurification catalyst interposed in an exhaust passage for purifyingexhaust gas; air-fuel ratio control means for controlling an in-cylinderair-fuel ratio of each cylinder; and ignition timing control means forcontrolling ignition timing of each cylinder, wherein the air-fuel ratiocontrol means increases a fuel injection amount at a first increaserate, during a first prescribed period corresponding to a period from atime point of end of the fuel cut to a time point of start of first fuelsupply with respect to each cylinder, in relation to a basic fuelinjection amount that is set based upon an operation state of theinternal combustion engine, and increases the fuel injection amount at asecond increase rate that is lower than the first increase rate, duringa second prescribed period following the first prescribed period, inrelation to the basic fuel injection amount; and the ignition timingcontrol means controls ignition timing with respect to the fuel suppliedduring the first prescribed period to be first retarded ignition timingthat is more retarded than reference ignition timing that is set basedupon the operation state of the internal combustion engine, and controlsignition timing with respect to the fuel supplied during the secondprescribed period to be second retarded ignition timing that is moreretarded than the reference ignition timing and more advanced than thefirst retarded ignition timing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are notlimitative of the present invention, and wherein:

FIG. 1 is a schematic construction view of a controller of an internalcombustion engine according to one embodiment of the present invention;

FIG. 2 is a block diagram showing an internal structure of an ECU in thecontroller of FIG. 1;

FIG. 3 is a time chart chronologically showing various states of anengine 1 at the time of fuel supply reversion control that isimplemented by the controller of FIG. 1; and

FIG. 4 is a characteristic graph of a first rich coefficient Ka that isused in the controller of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the attached drawings.

FIG. 1 is a schematic construction view of a controller of an internalcombustion engine according to one embodiment of the present invention.

An engine 1 (internal combustion engine) is an intake port injectiontype four-stroke in-line four-cylinder engine, which is constructed offour cylinders arranged in series and performs fuel ignition at eachregular intervals of 180 degrees crank angle. FIG. 1 shows a verticalsectional view of one of the cylinders. Illustration and descriptions ofthe other cylinders will be omitted as they have an identicalconstruction, and one cylinder will be representatively described here.

As illustrated in FIG. 1, the engine 1 is constructed so that a cylinderhead 4 is mounted on a cylinder block 2. A piston 12 is inserted into acylinder 10 formed in the cylinder block 2 so as to be verticallyslidable. The piston 12 is connected to a crank shaft 16 through aconnecting rod 14. The crank shaft 16 is provided with a crank anglesensor 18 for detecting crank angle.

A combustion chamber 20 is formed of the cylinder head 2, the cylinder10 and the piston 12. An ignition plug 22 is disposed in the cylinderhead 2 so that an electrode portion is exposed into the combustionchamber 20.

In the cylinder head 2, there is formed an intake port 24 thatcommunicates with the combustion chamber 20 and extends on one side ofthe engine 1 in width direction. A fuel injector 26 that injects fuelinto the intake port 24 is disposed so that the fuel injector 26 ispartially exposed into the intake port 24. The fuel injector 26 carriesout the fuel injection toward the inside of the intake port 24 accordingto pulse width of the fuel injection in signals supplied to the fuelinjector 24.

In the cylinder head 2, there is also formed an exhaust port 28 thatcommunicates with the combustion chamber 20 and extends on the otherside of the engine 1 in width direction.

Further disposed in the cylinder head 2 are an intake valve 30 thatperforms communication/disconnection between the combustion chamber 20and the intake port 24, and an exhaust valve 32 that performscommunication/disconnection between the combustion chamber 20 and theexhaust port 28.

An intake manifold 40 is connected to the one width-directional side ofthe engine 1 so as to communicate with the intake port 24. An intakepipe 42 is connected to an upstream-side end of the intake manifold 40.

A throttle valve 44 for adjusting an intake air amount of the engine 1is disposed in the intake pipe 42. Arranged in the upstream of thethrottle valve 44 is an air flow sensor (AFS) 46 for detecting theintake air amount. In the downstream of the throttle valve 44, there isprovided an intake pressure sensor 47 for detecting pressure in theintake manifold 40 (intake manifold pressure). An air cleaner 48 isdisposed in an end of the intake pipe 42.

An exhaust manifold 50 is connected to the other width-directional sideof the engine 1 so as to communicate with the exhaust port 28. Anexhaust pipe 52 is connected to the downstream-side end of the exhaustmanifold 50.

An exhaust gas purification catalyst 54 is disposed in the downstream ofthe exhaust pipe 52. The exhaust gas purification catalyst 54 is, forexample, a three way catalyst having a function for oxidizinghydrocarbon and carbon monoxide, and reducing NOx. The three waycatalyst contains noble metal such as Pt (platinum). The noble metal hasa property of adsorbing oxygen if oxidizing atmosphere continues andbeing degraded in NOx purification performance. An exhaust gas sensor 56for detecting oxygen concentration in exhaust gas is arranged in theupstream of an exhaust gas purification device 54.

In a vehicle equipped with the engine 1, there is disposed anaccelerator position sensor (hereinafter, referred to as APS) 58 fordetecting an operation amount of an accelerator, that is, acceleratordepression angle.

Various kinds of devices and sensors, including the APS 58, the crankangle sensor 18, the ignition plugs 22, the fuel injectors 26, thethrottle valve 44, the AFS 46, the exhaust gas sensor 56, etc., areelectrically connected to an ECU (electrical control unit) 60 (theair-fuel ratio control means and the ignition timing control means). TheECU 60 controls operations of the various devices according toinformation from the various sensors.

Specifically, in the engine 1, it is possible to perform so-called fuelcut that stops fuel supply to the engine 1 at the time of decelerationof the vehicle and the like. The ECU 60 has a function of implementingthe air-fuel ratio control and the ignition timing control of the engine1 at the time of reversion to a condition in which the fuel is suppliedto the engine 1 again to accelerate the vehicle or the like.

To be concrete, FIG. 2 shows a block diagram of an internal structure ofthe ECU 60, and input/output relationship of the ECU 60 will beexplained below with reference to FIG. 2.

Accelerator depression angle data that is detected by the APS 58 issupplied to a fuel-supply reversion judging portion 62 disposed in theECU 60. Engine revolution information that is calculated by a revolutioncalculating portion 63 based on output of the crank angle sensor 18 issupplied to the fuel-supply reversion judging portion 62. Based upon theaccelerator depression angle data and the engine revolution information,the fuel-supply reversion judging portion 62 makes a judgment as towhether the engine 1 returns from a fuel cut condition to the fuelsupply condition (hereinafter, occasionally referred to as reversionfrom the fuel cut). For example, when a press on the accelerator isreleased and the accelerator depression angle becomes zero at an enginerevolution equal to or more than prescribed revolution, a fuel cut flagis turned ON. In such a state, if the accelerator is pressed again, andthe accelerator depression angle grows larger, or if the enginerevolution is reduced to less than the prescribe revolution, the fuelcut flag is turned OFF and it is judged as reversion from the fuel cut.

When the reversion from the fuel cut is judged by the fuel-supplyreversion judging portion 62, the fuel-supply reversion judging portion62 outputs to a fuel-supply reversion control portion 64, data of aresult of the fuel-supply reversion judgment indicative that the engine1 returns from the fuel cut condition to the fuel supply condition.

Once the result of the fuel-supply reversion judgment is supplied to thefuel-supply reversion control portion 64, crank angle data that isdetected by the crank angle sensor 18 is first fed to a strokecalculating portion 66. The stroke calculating portion 66, based uponthe crank angle data, calculates the number of strokes of the engine 1from a time point of the reversion from the fuel cut. The stroke datathat is calculated by the stroke calculating portion 66 is supplied toan ignition timing compensating portion 68 (ignition timing controlmeans) and an air-fuel ratio compensating portion 70 (air-fuel ratiocontrol means) arranged in the fuel-supply reversion control portion 64.

Data of oxygen concentration in exhaust gas that is detected by theexhaust gas sensor 56 and intake air amount data that is detected by theAFS 46 are supplied to an exhaust gas air-fuel ratio calculating portion72 and an intake-air amount calculating portion 74 arranged in thefuel-supply reversion control portion 64. The exhaust gas air-fuel ratiocalculating portion 72 calculates an exhaust gas air-fuel ratio on thebasis of the oxygen concentration in the exhaust gas. The intake-airamount calculating portion 74 calculates an air amount that is suckedinto each cylinder, on the basis of the intake air amount data. Theexhaust gas air-fuel ratio data and the data of the intake air amountinto each cylinder are supplied to the air-fuel ratio compensatingportion 70.

Furthermore, a timer 76 is disposed in the fuel-supply reversion controlportion 64. The timer 76 detects elapsed time (or the number of strokes)from the time point of reversion from the fuel cut. Data of the elapsedtime (or the number of strokes) is also supplied to the air-fuel ratiocompensating portion 70. The air-fuel ratio compensating portion 70 isprovided with a counter for determining that the elapsed time (or thenumber of strokes) reaches prescribed time (or prescribed number ofstrokes).

The ignition timing compensating portion 68 to which the various kindsof data is supplied as described above compensates the ignition timingat the time of reversion from the fuel cut according to the variouskinds of data, and outputs to the ignition plug 22 an ignition signalcorresponding to the ignition timing that has been compensated. Theair-fuel ratio compensating portion 70 compensates the air-fuel ratio inthe cylinder at the time of reversion from the fuel cut according to thevarious kinds of data that has been supplied, and outputs a fuelinjection pulse signal corresponding to the compensated air-fuel ratioto the fuel injector 26.

Operation of the controller of an internal combustion engine accordingto the present embodiment thus constructed will be described below.

FIG. 3 is a time chart chronologically showing various states of theengine 1 at the time of fuel supply reversion control that isimplemented by the controller. The description will be provided belowwith reference to FIG. 3.

First, when the fuel cut flag is switched from ON to OFF, that is, whenthe engine 1 returns from the fuel cut condition to the fuel supplycondition, the fuel-supply reversion judging portion 62 disposed in theECU 60 determines this as reversion from the fuel cut, and thefuel-supply reversion control portion 64 implements the fuel supplyreversion control.

Specifically, the stroke calculating portion 66 and the timer 76 startcounting the number of strokes of the engine 1 and the elapsed time fromthe time point of reversion from the fuel cut.

During a time period when first fuel injection to each cylinder afterthe reversion from the fuel cut is carried out, that is, during firstprescribed period A corresponding to first four strokes, the air-fuelratio compensating portion 70 sets a pulse width of the fuel injectionso that a fuel supply amount markedly increases in relation to theintake air amount, and outputs the fuel injection pulse signal havingthis pulse width to the fuel injector 26.

To be more specific, the pulse width of the fuel injection at thismoment is a pulse width ka·INJ.PW obtained by multiplying a pulse widthINJ.PW serving as a base according to the intake air amount (forexample, a pulse width corresponding to a fuel injection amount formingstoichiometric air-fuel ratio) by a first rich coefficient Ka (firstincrease rate) that is set to a relatively high value in considerationof the fact that the cylinder is filled with fresh air.

The fuel injection carried out with the pulse width Ka·INJ.PW makes itpossible to realize the fuel injection in which a fuel injection amountis much higher than the fuel injection amount corresponding to astoichiometric air-fuel ratio according to the intake air amount. Thefirst rich coefficient Ka has a property of being increased as an intakemanifold pressure lowers at the time of reversion from the fuel cut asillustrated in FIG. 4, so that the first rich coefficient Ka is properlyvaried in accordance to an in-cylinder fresh-air amount that is changedby the intake manifold pressure.

The ignition timing compensating portion 68 sets the ignition timing ofignition with respect to an air-fuel mixture enriched by the first richcoefficient Ka, that is, the ignition timing of ignition from thirdstroke to sixth stroke after the reversion from the fuel cut, to a firstretarded ignition timing θa, and outputs an ignition signal according tothe first retarded ignition timing θa to the ignition plug 22. Inconsideration of the fact that the air-fuel mixture to be ignited isgreatly increased in fuel, the first retarded ignition timing θa isretarded to such ignition timing that there occurs a misfire at anair-fuel ratio that is set on the basis of a normal operation state (forexample, 20 degrees ATDC to 30 degrees ATDC).

As to second fuel injection for each cylinder, or fuel injection duringa second prescribed period B from fifth stroke, the air-fuel ratiocompensating portion 70 increases a fuel injection amount to higher thana fuel supply amount according to the intake air amount, and sets a fuelinjection pulse width that is compensated to be increased at a lowerincrease rate than that in the first prescribed period A. In otherwords, after the reversion from the fuel cut, when first combustion isgenerated in each cylinder, there exists remaining unburned gas in thecylinder as in the normal operation state. Therefore, if the fuelinjection amount is increased with the first rich coefficient Kaunchanged, there causes an overrich condition. For this reason, thepulse width of the fuel injection in the second prescribed period B isset to a pulse Kb·INJ.PW obtained by multiplying a second richcoefficient Kb (second increase rate) that is a value smaller than thefirst rich coefficient Ka by a base pulse width INJ.PW, on the basis ofa misfire limit of ignition timing retardation at the time when theremaining unburned gas exits in the cylinder. As is clear from FIG. 3,an amount of the fuel increase in the first prescribed period A is twiceor more than an amount of the fuel increase in the second prescribedperiod B.

As to the ignition with respect to an air-fuel mixture that is enrichedby the second rich coefficient Kb, or ignition of seventh and subsequentstrokes after the reversion from the fuel cut, the ignition timingcompensating portion 68 first sets ignition timing of seventh ignitionto second retarded ignition timing θb that is more retarded than theignition timing during the normal operation but more advanced than thefirst prescribed retarded ignition timing θa. To be concrete, the secondretarded ignition timing θb is set to the ignition timing that is soretarded as not to exceed the misfire limit of ignition timingretardation according to the second rich coefficient Kb (for example, 10degrees ATDC to 0 degree BTDC).

As to eighth and subsequent strokes, ignition timing is graduallyadvanced at prescribed gain from the second retarded ignition timing θbtoward reference ignition timing.

If it is determined that the elapsed time (or the number of strokes)that is counted by the timer 76 reaches the prescribed time in thecounter of the air-fuel ratio compensating portion 70, the air-fuelratio compensating portion 70 determines that timing for finishingenrichment is reached, and gradually reduces the amount of the fuelincrease at prescribed tailing gain in order to finish the enrichment ofthe air-fuel ratio, thereby leaning the air-fuel ratio toward thestoichiometric air-fuel ratio by degrees.

It is possible, instead of using the timer 76, to provide an exhaust gassensor 57 to an exhaust gas purification catalyst 54, monitor a state ofoxygen adsorption onto the exhaust gas purification catalyst 54 by useof the exhaust gas sensor 57, and start the leaning with an output ofthe exhaust gas sensor 57 as a trigger.

It is also possible to so control as to figure out an oxygen amount thatis adsorbed on the exhaust gas purification catalyst 54 by calculating achanged amount of the exhaust gas air-fuel ratio and an air amount thatpasses through the exhaust gas purification catalyst 54 by means of theexhaust gas sensor 56 and the AFS 46 and by multiplying the exhaust gasair-fuel ratio changed amount and the catalyst passing air amount, andto start the leaning of the air-fuel ratio with a time point when theoxygen adsorption amount is reduced to equal to or less than aprescribed amount as enrichment finishing timing. In this case,according to the present embodiment, the oxygen amount that is adsorbedonto the exhaust gas purification catalyst 54 is calculated in the ECU60. Therefore, the ECU 60 corresponds to oxygen-adsorption amountestimation means of the present invention.

The enrichment finishing timing may be decided according to both thetimer 76 and the calculated oxygen adsorption amount. If the enrichmentfinishing timing is set according to the oxygen adsorption amount inthis manner, the enrichment can be completed with more proper timing.

In case that the engine returns from the fuel cut condition to the fuelsupply condition, since the engine is in the fuel cut conditionimmediately before the first prescribed period A, there exists noremaining unburned gas but only fresh air in each cylinder in the firstprescribed period A. Accordingly, if the fuel supply amount is greatlyincreased in relation to the intake air amount as stated, the exhaustsair-fuel ratio can be enriched to a great degree immediately after thefuel cut is finished while suppressing an increase in the dischargeamount of hydrocarbon into the atmosphere. By the substantial enrichmentof the exhaust gas air-fuel ratio immediately after the finish of thefuel cut, the oxygen that has adsorbed onto the exhaust gas purificationcatalyst during the fuel cut can be promptly consumed while discharge ofNOx is suppressed, which efficiently prevents the increase of thedischarge amount of NOx into the atmosphere.

The substantial fuel increase in the first prescribed period A shiftsthe misfire limit in a retardation direction, which expand an area inwhich no misfire occurs. This makes it possible to retard the ignitiontiming to the first retarded ignition timing θa that normally causes amisfire. If the ignition timing is drastically retarded as describedabove, it is possible to prevent a sudden change in shaft output at thetime of the reversion from the fuel cut, to thereby reduce shock.

In the second prescribed period B where there exists the remainingunburned gas in each cylinder, if the fuel is increased at a lowerincrease rate than that in the first prescribed period A, it is possibleto suppress an increase in the discharge amount of HC into theatmosphere while maintaining the enrichment of the exhaust gas air-fuelratio, and then to keep suppressing the discharge of NOx into theatmosphere. In the second prescribed period B, the ignition timing isset to the second retarded ignition timing θb that is more advanced thanthe first retarded ignition timing θa, and is then advanced in stages.This makes it possible to efficiently actualize the reduction of theshock while preventing a misfire at the same time.

Furthermore, if the in-cylinder air-fuel ratio is gradually leaned froma time point when the second prescribed period B is over, the timingwhen the ignition timing is gradually advanced and the timing when theair-fuel ratio is gradually leaned are staggered from each other, andthe shock can be reduced more effectively.

As described above, the controller of an internal combustion engineaccording to the present embodiment is capable of efficientlysuppressing the increase of the discharge amount of NOx into theatmosphere at the time of reversion from the fuel cut, and of reducingthe shock caused at the time of the reversion from the fuel cut.

The description of the controller of an internal combustion engineaccording to the one embodiment of the present invention is finishedhere, but an aspect of the embodiment is not limited to the foregoingembodiment.

For example, in the above embodiment, the fuel-supply reversion judgmentis made according to the information from the APS 58 or enginerevolution information, and the calculation of the number of strokes iscarried out based on the information from the crank angle sensor 18. Thecalculation of the exhaust gas air-fuel ratio is performed based on theinformation from the exhaust sensor 56, and the calculation of theintake air amount is made based on the information from the AFS 46.However, these judgment and calculations may be carried out according toinformation from the other means.

Although the engine 1 is a manifold injection-type engine in the aboveembodiment, the engine is not limited thereto, but may be, for example,an in-cylinder injection type engine.

The invention thus described, it will be obvious that the same may bevaried in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A controller of an internal combustion engine capable of carrying outfuel cut and stopping fuel supply into each cylinder, said controllercomprising: an exhaust gas purification catalyst interposed in anexhaust passage for purifying exhaust gas; air-fuel ratio control meansfor controlling an in-cylinder air-fuel ratio of each cylinder; andignition timing control means for controlling ignition timing of eachcylinder, wherein: said air-fuel ratio control means increases a fuelinjection amount at a first increase rate, during a first prescribedperiod corresponding to a period from a time point of end of said fuelcut to a time point of start of first fuel supply with respect to eachcylinder, in relation to a basic fuel injection amount that is set basedupon an operation state of said internal combustion engine, andincreases the fuel injection amount at a second increase rate that islower than said first increase rate, during a second prescribed periodfollowing said first prescribed period, in relation to said basic fuelinjection amount; and said ignition timing control means controlsignition timing with respect to the fuel supplied during said firstprescribed period to be first retarded ignition timing that is moreretarded than reference ignition timing that is set based upon theoperation state of said internal combustion engine, and controlsignition timing with respect to the fuel supplied during said secondprescribed period to be second retarded ignition timing that is moreretarded than said reference ignition timing and more advanced than saidfirst retarded ignition timing.
 2. The controller of an internalcombustion engine according to claim 1, wherein: the lower an intakemanifold pressure at the time point of the end of said fuel cut is, thehigher said first increase rate is set.
 3. The controller of an internalcombustion engine according to claim 1, wherein: said ignition timingcontrol means sets the ignition timing with respect to the fuel suppliedduring said second prescribed period to said second retarded ignitiontiming and thereafter gradually advances the ignition timing toward saidreference ignition timing.
 4. The controller of an internal combustionengine according to claim 3, wherein: said air-fuel ratio control meansgradually reduces a fuel injection amount of said each cylinder towardsaid basic fuel injection amount, immediately after the end of saidsecond prescribed period.
 5. The controller of an internal combustionengine according to claim 1, further comprising: oxygen-adsorptionamount estimation means for estimating an adsorption amount of oxygenthat is adsorbed on said exhaust gas purification catalyst, wherein:said second prescribed period ends at a time point when theoxygen-adsorption amount that is estimated by said oxygen-adsorptionamount estimation means is reduced to equal to or less than a prescribedamount.
 6. The controller of an internal combustion engine according toclaim 1, wherein: a fuel increase amount based upon said first increaserate is twice or more than a fuel increase amount based upon said secondincrease rate.
 7. The controller of an internal combustion engineaccording to claim 1, wherein: said basic fuel injection amount is aninjection amount corresponding to a stoichiometric air-fuel ratio.